1. Field of the Invention
This invention relates generally to the field of measuring and marking tools having a generally triangular configuration and a flange, perpendicular to one side, projecting above and below the upper and lower surfaces of the triangle, and upon which linear measurement markings are provided on the surfaces of the triangle along the respective edge portions. In particular, this invention relates to tools having a scalene triangular configuration in which the angular orientation of the sides relative to the flange are common angles used in mitered construction and markings on a surface of the triangle providing commonly used mitered air duct joint fabrication measurements.
The use of fibrous ductboard material for heating and air conditioning ducts is well known. Such ductboard typically includes a layer of fiberglass attached to a composite outer covering. The outer covering is typically made up of a layer Kraft paper, a layer of scrim-like material, and a foil-like layer, the composition providing stiffness and forming an air-impervious outer layer for the duct. Fibrous ductboard material is commonly available in flat-sheet form or pre-formed into rectangular cross-section ducts in a variety of sizes. When flat-sheet material is used, it is conventional to cut, either by machine or the installer, a set of laterally-spaced-apart, longitudinal grooves in the fiberglass side of flat-sheet ductboard to form rectangular cross-section duct sections.
It is also well known in the duct construction field that fittings, such as elbows and offsets, may be fabricated from the ductboard material using mitered cuts on the duct. For example, a simple 90-degree elbow can be formed by cutting the duct along a plane oriented 45 degrees to the longitudinal duct axis, rotating one segment one-half turn about its longitudinal axis, and connecting the two sections at the cut plane to form a 90-degree mitered elbow. Fittings having more gradual transitions, thereby imposing less resistance on air flow, are commonly formed by reducing the miter angle and increasing the number of duct segments comprising the fitting.
In preparation for making a duct fitting, the user assembles tools including a marking pencil, an incrementally marked straight edge, a protractor, and a cutting knife and determines the dimensions of the desired fitting. The user then makes a series of cutting marks, or layout lines, on the ductboard material to define a cutting plane. Conventional flaming squares or carpenter's triangles are typically employed to make layout lines oriented perpendicular to the longitudinal axis of the duct. Non-perpendicular layout lines require the user to establish two points along the desired line with a protractor or other similar tool and then make a line using a straight edge. The duct can then be cut by drawing a cutting knife along the layout line, optionally using a straight edge as a guide, and cutting all the way thorough the ductboard material. Finally, the user aligns the resulting duct segments to form the desired fitting, reconnects the segments with adhesive, and seals the connection with duct tape. Drawbacks in using the above-described tools and method for making mitered elbows and offsets are the considerable length of time involved and the limited quality and accuracy of the resulting joint due to variations in measuring and cutting.
It is therefore the principle objective of the present invention to provide a tool and a method for selecting and making a series of quick layout lines for the most common miter angles used in air duct fitting fabrication and for aligning the edge of a cutting tool in making the cuts. It is another objective of the present invention to provide a tool and a method for controlling the accuracy and quality of miter joint cuts.
2. Description of Related Art
Numerous measuring and marking tools of the right-triangular type are known in the prior art. U.S. Pat. No. 4,513,510, by Swanson, discloses a right-triangular-shaped layout tool with a T-flange base on one side, and which is adaptable with a layout bar to provide a means for repeated marking of predetermined angles as are common in marking of stair stringer boards. U.S. Pat. No. 5,727,325, by Mussell, discloses a right-triangular-shaped tool with a T-flange on one side and markings to facilitate aligning the tool on workpieces at selected angles commonly used in rafter and stair stringer framing. U.S. Pat. No. 6,622,394, by Werner, discloses a right-triangular-shaped measuring tool with a T-flange for aligning the tool base to the workpiece. Indicia along the hypotenuse in conjunction with a defined origin allow marking of acute angles commonly used in deck construction. U.S. Pat. No. 6,688,014, by Allemand, discloses a right-triangular-shaped measuring and marking tool that includes internal structures for marking frequently used wood frame construction dimensions and a method of using the tool to mark layout lines common in wood frame construction.
These measuring and marking tools have limited efficacy compared with the present invention. All are based on a right triangle having a flange perpendicular to one leg of the triangle useful for rapidly aligning the tool with an edge of the workpiece. The limitation with this right-triangular design is that the second leg of the right triangle is always perpendicular to the flange, leaving only the hypotenuse available for non-perpendicular layout lines. To overcome this limitation, each tool includes structures for marking other common framing angles. However, using these features is a multi-step operation. One method requires making a pair of marks to define a line, moving the tool, and using a straight edge to draw the desired line. Another method requires visually aligning two or more points on the tool with a reference edge of the workpiece to establish the desired angle and then drawing the desired line. In the former method, making a layout line is a three-step process; the second method requires two steps and fails to take advantage of the T-flange for quick and accurate tool alignment with the workpiece.
In addition to framing tools, drawing instruments are known in the prior art. U.S. Pat. No. 2,610,407, by McQuaid, discloses a drafting instrument incorporating straight edges inclined at angles commonly used in making axonometric projection drawings. U.S. Pat. No. 4,455,760, by Arceneaux, discloses a drafting instrument incorporating straight edges inclined at angles commonly used in making isometric projection drawings. These instruments depart from a standard right-triangular-shaped design and incorporate interior structures thereby increasing the number of straight edges offered in a single instrument. As drafting instruments, these instruments do not incorporate a T-flange to align the instrument against a workpiece corner. Alignment to a reference line is commonly performed using a T-square or similar drafting apparatus.
Layout and fabrication tools specifically useful for working with fibrous ductboard material are also well known in the prior art. U.S. Pat. No. 4,179,808, by Smith, discloses a movable tool guide for cutting and removing wedge-shaped pieces from sheet-form fibrous ductboard material enabling air duct transition pieces of a range of sizes to be formed. U.S. Pat. No. 4,608,902, by Ivey, discloses a portable measuring and cutting tool guide for cutting parallel V-grooves in fibrous ductboard material enabling rectangular air duct to be formed from flat-sheet material. These tools are designed to make layout lines and cuts on ductboard material in sheet form needed to form rectangular cross-section ducts, but they are not suited to working with pre-formed rectangular cross-section ducts.